Establishment and Dispersal ofUrophora affinis(Diptera: Tephritidae) andMetzneria paucipunctella(Lepidoptera: Gelechiidae) in Southwestern Virginia

Two European gall-producing insects,Urophora affinisFrfld. (Diptera: Tephritidae) andMetzneria paucipunctella(Zeller) (Lepidoptera: Gelechiidae) were introduced into Virginia in 1986 for biological control of spotted knapweed (Centaurea maculosaLam.). Adults ofU. affinis(n = 2625) andM. paucipunctella(n = 450) were released at two sites in Montgomery County, Virginia, and their populations were monitored yearly by dissecting spotted knapweed flower heads. Beginning in 1992, knapweed samples collected at various distances from the release sites were checked for dispersal.U. affinisis well established and is spreading slowly. The number of larvae per flower head and the seed numbers are inversely related as plants with the greatest number of larvae per spotted knapweed head had the lowest number of seeds. Knapweed density has declined at one of the release sites which had the highest rate of infestation byU. affinis.Establishment of the moth,M. paucipunctella,is less certain as it has been recovered at a very low level from only one site.     

Role of plant phenology in mediating interactions between two biological control agents for spotted knapweed

The role of spotted knapweed phenology on the attack rate of two seed-head insects Urophora affinis and Larinus minutus was assessed in a series of field studies at four study sites in south-eastern British Columbia, Canada. Slow or later developing knapweed plants had more seed heads that contained only single or multiple U. affinis whereas early or faster developing plants had more seed heads containing L. minutus alone or in combination with U. affinis. L. minutus did not distinguish between seed heads with or without U. affinis larvae when laying eggs. However, seed heads with multiple U. affinis present, produced fewer L. minutus adults than expected. The probability of single or multiple U. affinis galls being present increased with seed-head diameter but was not affected by seed-head height. Attack by L. minutus increased with seed-head diameters >5 mm and was lower at plant heights above 50 cm. These results demonstrate two mechanisms that enable U. affinis to successfully coexist with L. minutus: differences between the species in their response to the developmental phenology of knapweed heads, and increased survivorship of U. affinis in heads with multiple U. affinis galls through niche interference competition. These mechanisms provide a possible explanation for the persistence of U. affinis populations on spotted knapweed, in spite of high levels of within seed-head mortality that have been observed with increasing L. minutus populations.     

Supercooling capacity of Urophora affinis and U. quadrifasciata (Diptera: Tephritidae) on spotted knapweed: comparisons among plants, sites, time of season, and gall densities

Larval supercooling points of Urophora affinis Frauenfeld and U. quadrifasciata (Meigen) were compared among plants, six research sites in western Montana, four fall/winter time periods, and among gall densities. These two tephritid fly species are introduced biological control agents of spotted knapweed, Centaurea maculosa Lamarck, and diffuse knapweed, Centaurea diffusa Lamarck. Few differences in larval supercooling points for U. affinis and U. quadrifasciata were found among plants, and where differences were found, they were not consistent across fall/winter time periods. Significant differences in larval supercooling points were found among sites and across fall/winter time periods. No relationship was found between larval supercooling points and site elevation. Larval supercooling points of both U. affinis and U. quadrifasciata showed no relationship with the density of Urophora galls within spotted knapweed capitula. Mean larval supercooling points of U. affinis were consistently lower than those of U. quadrifasciata across sites and fall/winter time periods. In conclusion, temporal differences in temperature over the fall/winter time periods and microclimatic differences among sites appear to be the most important abiotic factors influencing the supercooling points in U. affinis and U. quadrifasciata.     

Field cage assessment of interference among insects attacking seed heads of spotted and diffuse knapweed

Field studies were conducted to determine the competitive interactions between introduced biological control agents that attack the seed heads of spotted knapweed (Centaurea stoebe ssp. micranthos) and diffuse knapweed (Centaurea diffusa). Two weevils, Bangasternus fausti and Larinus minutus (Coleoptera: Curculionidae), were each paired with the previously established fly, Urophora affinis (Diptera: Tephritidae). Each species was released either alone or in pair-wise combinations inside screen cages placed over existing knapweed plants at six field sites in Montana and one in Oregon. Larinus minutus produced almost three times as many progeny on diffuse knapweed as on spotted knapweed. Larinus minutus reproduction was not affected by competition with U. affinis, but U. affinis reproduction was reduced by the presence of L. minutus (by 71% on spotted and 77% on diffuse knapweed). Bangasternus fausti reproduction generally was not affected by competition with U. affinis, nor was U. affinis affected by B. fausti on either host plant. There were extremely few cases of successful production of both weevil and fly in the same capitulum, which was probably because weevil larvae consume the developing flies. Both weevils increased the total proportion of seed heads infested on diffuse knapweed, and B. fausti increased it on spotted knapweed. However, the release of either weevil did not significantly further reduce seed production on either plant. The results and experimental design are discussed in light of the subsequent establishment and impact of these agents.     

Interactions and effects of multiple biological control insects on diffuse and spotted knapweed in the Front Range of Colorado

Abundances and interactions among biological control insects and their effects on target invasive plants were monitored within the flower heads and roots of diffuse knapweed, Centaurea diffusa, and in spotted knapweed, Centaurea stoebe, along the Colorado Front Range. Flower weevils, (Larinus species) and root-feeders (Cyphocleonus achates and Sphenoptera jugoslavica) were released on knapweed that already supported biological control gall flies (Urophora species). At a single monitoring site, seed production by C. diffusa declined from 4400 seeds m⁻² in 1997 to zero seeds m⁻² on the monitoring sites in 2006, while the flowering stem density of C. diffusa declined from a peak of almost 30 stems m⁻² in 2000 to zero stems m⁻² in 2006. The average abundance of Urophora and Larinus in flower heads fluctuated independently during the 2001–2006 interval, while the relative abundance of C. achates and S. jugoslavica in roots exhibited a weak inverse relationship that appeared driven by climate effects. The relative abundance of insects on a population of C. stoebe was monitored for five years as Larinus species and C. achates became established on spotted knapweed that already supported Urophora species. Spotted knapweed seed production on our monitoring site declined from 4600 seeds m⁻² in 2003 to zero seeds m⁻² in 2006. Unlike C. diffusa, substantial numbers of rosettes of C. stoebe remained present. Larinus consumed almost all Urophora encountered in C. diffusa, and consumed about 40% of the Urophora in co-infested flower heads of C. stoebe (ca. 10–15% of the total Urophora population). No negative correlations between the relative densities of flower head and root-feeding insects were observed. The effects of these insects on target plants have produced results consistent with the ‘cumulative stress hypothesis’ for biological control of Centaurea species.     

Non-target effects of an introduced biological control agent on deer mouse ecology

Release of exotic insects as biological control agents is a common approach to controlling exotic plants. Though controversy has ensued regarding the deleterious direct effects of biological control agents to non-target species, few have examined the indirect effects of a ”well-behaved” biological control agent on native fauna. We studied a grassland in west-central Montana infested with spotted knapweed (Centaurea maculosa) to examine the effects of knapweed invasion and two gall flybiological control agents (Urophora affinis and U. quadrifasciata) on the native deer mouse (Peromyscus maniculatus). Stomach-content analysis revealed that Urophora were the primary food item in Peromyscus diets for most of the year and made up 84–86% of the winter diet. Stomach contents indicated that wild-caught mice consumed on average up to 247 Urophora larvae mouse^–1 day^–1, while feeding trials revealed that deer mice could depredate nearly 5 times as many larvae under laboratory conditions. In feeding trials, deer mice selected knapweed seedheads with greater numbers of galls while avoiding uninfested seedheads. When Urophora larvae were present in knapweed seedheads, deer mice selected microhabitats with moderately high (31–45% cover) and high knapweed infestation (≥46% cover). After Urophora emerged and larvae were unavailable to Peromyscus, mice reversed habitat selection to favor sites dominated by native-prairie with low knapweed infestation (0–15%). Establishment of the biological control agent, Urophora spp., has altered deer mouse diets and habitat selection by effecting changes in foraging strategies. Deer mice and other predators may reduce Urophora populations below a threshold necessary to effectively control spotted knapweed. Received: 04 May 1999 / Accepted: 14 August 1999     

The lesser of two weevils: physiological responses of spotted knapweed (Centaurea stoebe) to above- and belowground herbivory by Larinus minutus and Cyphocleonus achates

The physiological responses of plants to variable levels of root and shoot herbivory in the field may yield valuable insights regarding potential compensation or tolerance responses for herbivory. In an infestation of Centaurea stoebe (spotted knapweed) located in the Colorado foothills, we measured physiology, biomass, and flower production of individual plants undergoing a natural range of herbivory by the above- and belowground biological control insects, Larinus minutus and Cyphocleonus achates. Over the growing season, net carbon assimilation rate, transpiration, stomatal conductance, and intercellular leaf [CO₂] (C _i) all decreased, while water use efficiency increased. The decrease in these physiological traits was due to an increase in the intensity of L. minutus damage over time; effects of C. achates root damage to plant physiology, including transpiration were only marginally significant. The effects of these two species on plant physiology were not interactive, and Larinus minutus was found to exert larger negative effects on this invasive plant in terms of plant physiology and potential reproductive output than C. achates. While previous studies have shown C. achates to have significant negative effects on population densities of spotted knapweed, the addition of Larinus minutus to the suite of insects used in biological control of spotted knapweed should facilitate continued or enhanced reduction in densities of this noxious weed.     

Effect of summer drought relief on the impact of the root weevil Cyphocleonus achates on spotted knapweed

A recent decline in spotted knapweed, Centaurea stoebe L. subsp. micranthos (Asteraceae), has been observed in parts of western Montana. The release of the biological control agent Cyphocleonus achates (Fahraeus) is thought to contribute to the decline, but persistent drought since at least 1999 may be an additional factor. We conducted outdoor plot experiments to test the relative impacts of C. achates weevils and summer drought relief on spotted knapweed survival and growth. Groups of spotted knapweed transplants were assigned to one of four weekly water addition treatments (no added water, and 0.25, 0.5 or full recovery of plant water deficit, where "deficit" refers to potential evapotranspiration minus rainfall) in May to August 2004 and June to August 2005 and to either exposure to or protection from C. achates. In June of each subsequent year (2005 and 2006), plants were harvested and growth attributes that reflect plant vigor were measured. Drought indices showed that throughout the time of the study until January 2006, western Montana was in drought alert or severe drought. Summer drought relief had no effect on aboveground biomass and plant height of knapweed plants in subsequent years, but feeding by C. achates larvae reduced these two measures of plant vigor. Knapweed plants resuming growth after the drought ended in spring 2006 were significantly larger than those resuming growth under drought conditions in spring 2005. Spring drought may reduce knapweed growth, but C. achates reduced knapweed growth regardless of drought conditions.     

How do biological control and hybridization affect enemy escape?

Two mechanisms often linked with plant invasions are escape from enemies and hybridization. Classical biological control aims to reverse enemy escape and impose top-down population control. However, hybridization has the potential to alter interactions with enemies and thus affect biological control. We examined how introductions of biological control agents affect enemy escape by comparing specialist enemy loads between the native and introduced ranges of two noxious weeds (spotted and diffuse knapweed; Centaurea stoebe L. and C. diffusa Lam.) that have been the targets of an extensive biological control program. Hybrids between spotted and diffuse knapweed are often found within diffuse knapweed sites in North America, so we also compared enemy loads on plants that appeared morphologically like diffuse knapweed and hybrids. Finally, we tested the preference for diffuse knapweed, hybrids, and spotted knapweed of one of the agents thought to be instrumental in control of diffuse knapweed (Larinus minutus; Curculionidae). In North America spotted knapweed has largely escaped its root herbivores, while seedhead herbivore loads are comparable in the introduced and native ranges. Diffuse knapweed exhibited seedhead herbivore loads five times higher in the introduced compared to native range. While this pattern of seedhead herbivory is expected with successful biological control, increased loads of specialist insect herbivores in the introduced range have rarely been reported in the literature. This finding may partially explain the better population control of diffuse vs. spotted knapweed. Within North American diffuse knapweed sites, typical diffuse knapweed and hybrid plants carried similar herbivore loads. However, in paired feedings trials, the specialist L. minutus demonstrated a preference for newly created artificial hybrids over North American diffuse knapweed and for European diploid spotted knapweed over North American tetraploid spotted knapweed. Overall though, hybridization does not appear to disrupt biological control in this system.     

Effects of plant competition,seed predation,and nutrient limitation on seedling survivorship of spotted knapweed (<Emphasis Type="Italic">Centaurea stoebe</Emphasis>)

We measured seed germination and seedling survivorship of spotted knapweed, Centaurea stoebe, in a series of laboratory and field experiments to evaluate the efficacy of seed limitation as a management focus. This work was initiated 6 years after introduction of several biological control agents. The soil seed bank of the site used in this study contained a mean density of 5,848 seeds/m² (ranging from 0 to 16,364 seeds/m²), and 92% of the seeds isolated from soils were shriveled, discolored, and/or partially decayed. Additionally, none of the intact seeds germinated, suggesting that the viable seed bank at our field study site has been exhausted. Centaurea stoebe seeds were planted into pots under a range of soil nitrogen (N) availability, with half of the pots containing a single density of previously established seedlings of a native cool-season grass, slender wheatgrass (Elymus trachycaulus). A watering regime mimicking local precipitation was applied. Spotted knapweed exhibited large biomass responses to N addition, but the presence of grasses suppressed the ability to exploit this N. Surprisingly, low soil N conditions improved knapweed survivorship in the presence of grasses. Nevertheless, recruitment and biomass were still far below the levels reached in the absence of competition. To evaluate the effect of density on successful recruitment, Centaurea stoebe seed was introduced into a meadow at three densities matching reduced levels of seed production under the constraints of seed predators. These densities were sown with or without a seed mixture of native species, into an existing plant community lacking C. stoebe, and seedling recruitment was recorded over 2.5 years. Across all plots and densities sown (568–2,272 seeds m⁻² year⁻¹), seedling recruitment was less than 1%. The invasion potential of spotted knapweed was greatly diminished when realistic levels of plant competition and biological control limit seed production. We therefore conclude that a combination of seed limitation and shortage of ‘safe sites’ within undisturbed vegetation can limit densities of C. stoebe.     

Biological control agents elevate hantavirus by subsidizing deer mouse populations

Biological control of exotic invasive plants using exotic insects is practiced under the assumption that biological control agents are safe if they do not directly attack non-target species. We tested this assumption by evaluating the potential for two host-specific biological control agents ( Urophora spp.), widely established in North America for spotted knapweed ( Centaurea maculosa ) control, to indirectly elevate Sin Nombre hantavirus by providing food subsidies to populations of deer mice ( Peromyscus maniculatus ), the primary reservoir for the virus. We show that seropositive deer mice (mice testing positive for hantavirus) were over three times more abundant in the presence of the biocontrol food subsidy. Elevating densities of seropositive mice may increase risk of hantavirus infection in humans and significantly alter hantavirus ecology. Host specificity alone does not ensure safe biological control. To minimize indirect risks to non-target species, biological control agents must suppress pest populations enough to reduce their own numbers.     

Impact of clear-cutting and prescribed burning on microbial diversity and community structure in a Jack pine (Pinus banksiana Lamb.) clear-cut using Biolog Gram-negative microplates

Three isolates ofFusarium avenaceum are pathogenic on spotted knapweed(Centaurea maculosa), a major weed plant of pasturelands and rangelands of the Pacific Northwestern USA. One isolate (no. 1) obtained from the European centre of origin of knapweed and isolate no. 365 native to Montana, did not significantly affect knapweed seed germination. However,F. avenaceum no. 1003, another Montana native isolate, caused a 100% decrease in seed germination and hence, no seedling emergence. When formulated, isolate no. 1003, could be recovered from treated soils after 7 days and caused a significant reduction in seedling emergence or seedling dry weight. This organism had no effect on the germination ofTriticum aestivum orMedicago sativa, but did affect the germination of other plant species.F. avenaceum appears to be a candidate for the biocontrol of spotted knapweed, however, a native isolate is potentially more effective than an isolate obtained from the centre of origin ofC. maculosa.     

Arbuscular mycorrhizal fungi enhance spotted knapweed growth across a riparian chronosequence

Arbuscular mycorrhizal fungi (AMF) mediate nutrient uptake that accelerates plant growth and reproduction. Thus, AMF may promote plant invasions often observed along rivers. We assessed the importance of AMF in improving growth of the invasive species, spotted knapweed (Centaurea stoebe), during succession of riparian vegetation along a flood plain in Montana, USA. We grew spotted knapweed with and without AMF in soils collected from riparian sites ranging from 1 to 72 years old and measured the plant’s growth response to AMF. We observed variability in relative effects of AMF, with greatest growth benefits in recently deposited alluvial sediments. We then separated effects of soil and inoculum source by growing spotted knapweed with soils and inocula collected from young or old sites and found that growth responses were greatest in young soils regardless of inoculum source. Our results demonstrate that AMF directly benefit growth of spotted knapweed, especially in soils that typify early successional sites on this alluvial flood plain.     

New techniques and findings in the study of a candidate allelochemical implicated in invasion success

Allelopathy has been hypothesized to promote the success of invasive plants. Support for the role of allelopathy in invasions has emerged from research on the candidate allelochemical (?)‐catechin, which is secreted by spotted knapweed. Here we describe new methods to quantify catechin in liquid and soil. With a new technique, we assayed catechin production by individual plants in liquid media and found levels up to two orders of magnitude less than previously reported. An acetone/water solution provided consistent recovery of catechin from soil, with percent recovery depending upon soil type. We evaluated soils from two spotted knapweed sites in Montana, USA, but found no measurable catechin. Idaho fescue, a native species reportedly sensitive to catechin, only exhibited slightly reduced growth at concentrations 10 times higher than previously reported to cause 100% mortality. Our results emphasize that more research is required to clarify the role of catechin in the invasion of spotted knapweed.     

Interspecific interactions between the gall-fly Urophora affinis Frfld. (Diptera: Tephritidae) and the weevil Larinus minutus Gyll. (Coleoptera: Curculionidae), two biological control agents released against spotted knapweed, Centaurea stobe L. ssp. micranthos

Interspecific competition has been suggested as an explanation for the failure of some insects as biological control agents for weeds. Enclosure and exclusion cages were used, in southern British Columbia, Canada to evaluate the importance of interspecific competition between a seedhead weevil, Larinus minutus, and a gall-inducing fly, Urophora affinis, two biocontrol agents released against spotted knapweed in North America. At the seedhead scale, U. affinis, which is an inferior biological control agent based on knapweed seed mortality, was the superior competitor. Larinus minutus attack rates were significantly lower in the presence of U. affinis compared to release treatments where L. minutus was attacking alone. Reduced L. minutus attack rates were apparent in seed heads expected to contain both species, assuming insect distributions were random, but instead only contained U. affinis. L. minutus did not significantly affect U. affinis density. Although overall attack rates on knapweed seedheads were higher when both species were together at a site, the consequence of the antagonistic interaction is that overall seed destruction was not as high as it could have been if the weevil were attacking on its own. These results support minimizing the number of biocontrol agents released that use similar resources on the target weed, to avoid negative interactions between control agents and potential reductions in biocontrol efficacy resulting from competitive exclusion.     

Phenology and dispersal of Larinus minutus Gyllenhal and Larinus obtusus Gyllenhal (Coleoptera: Curculionidae), two biological control agents of Centaurea stoebe ssp. micranthos (spotted knapweed) in Michigan

Centaurea stoebe L. ssp. micranthos (Gugler) (spotted knapweed) is an invasive plant that has been the target of classical biological control in North America for more than four decades. The seedhead-feeding weevils Larinus minutus Gyllenhal and Larinus obtusus Gyllenhal (Coleoptera: Curculionidae) are two of the most-widely released C. stoebe control agents, and have more recently been introduced into the eastern US. While there have been many studies focusing on their ability to impact C. stoebe in the western US and Canada, there have been few studies from eastern North America, and basic knowledge of important aspects of their biology is lacking. Here we report on the phenology and dispersal of L. minutus and L. obtusus in Michigan. We regularly sampled two established Larinus spp. populations in southern Michigan in 2012 and 2013, and found that while adult abundance fluctuates during the growing season, they remained at easily detectable levels from mid-June through the end of August. We also used previously established populations of L. minutus and L. obtusus released in 1996 (n = 1), 2007 (n = 2), and 2010 (n = 5) to determine how dispersal of Larinus spp. into the surrounding landscape changes with time since release. Populations of Larinus spp. weevils showed little dispersal for 2 years post-release. However, after initial establishment dispersal rates increased rapidly, resulting in average dispersal rates that increased exponentially with time since release. These findings can inform future biological control release and sampling programs for Larinus spp. in eastern North America.     

内蒙古锡林郭勒克氏针茅退化草原土壤种子库特征

以内蒙古锡林郭勒地区克氏针茅(Stipa krylovii)草原不同退化等级群落为对象,研究克氏针茅退化草原可萌发土壤种子库特征.结果表明,随着草原退化程度的增加,不论是土壤总种子库还是持久土壤种子库,组成和密度均明显下降, 重度和极度退化草原土壤总种子库密度下降至仅为轻度退化草原的46.8%和11.1%.代表土壤总种子库的4月份取样, 轻度、中度、重度和极度退化草原各样地0～9 cm土壤种子库密度分别为2800、1278、1311和311粒·m-2;代表持久土壤种子库的6月底取样,4个样地土壤种子库密度分别为1667、967、334和167粒·m-2.多数植物土壤种子库主要分布在0～6 cm土层,各样地种子库密度随土壤深度的增加而减少,轻度、中度、重度和极度退化草原4月份0～6 cm土层种子库分别占总种子库(0～9 cm)的98.4、96.5、95.8 和85.7%.不同退化等级草原地上植被与土壤种子库的Sorensen相似性指数介于0.24～0.48.     

Distribution of Insect Attacks in Biological Control of Weeds: Infestation of Centaurea virgata Flowerheads by a Gall Fly

The success of biological control efforts to reduce weed density through release of insects may depend as much on the distribution of insect attacks among individual plants or plant parts as on the mean level of infestation. We used an index of dispersion to describe the distribution of Urophora quadrifasciata (Diptera: Tephritidae) galls among squarrose knapweed (Centaurea virgata) flowerheads at 18 west central Utah sites in the first 5 years following introduction of the biological control agent. Two thirds of the samples showed a significantly aggregated distribution of galls among flowerheads. Statistical analysis showed that site and year accounted for relatively small proportions of the variance in the index of dispersion. The degree of gall aggregation among flowerheads was positively correlated with the mean flowerhead quality (mean number of seeds per flowerhead; P = 0.013) and tended to be negatively correlated with the mean fly density per flowerhead at a site in a given year (P = 0.097). Our data suggest that higher quality flowerheads, and possibly higher quality plants, are preferentially attacked by U. quadrifasciata and therefore are more heavily subject to reduced reproductive potential through biological control. However, an aggregated distribution of fly attacks may undercut the potential of the fly to reduce seed production by the weed population as a whole. Understanding both the distribution of insect attacks among individual plants and the behavioral mechanisms producing such distribution patterns is important to the biological control of weeds.     

Diffuse knapweed (Centaurea diffusa Lam.) seedling emergence and establishment in a Colorado grassland

Knapweeds (Centaurea spp.) are damaging invaders of grasslands and other North American rangelands. A field study was conducted to determine conditions that promote diffuse knapweed (C. diffusa) emergence and establishment in a native Colorado grassland (North America). Knapweed was planted in native grassland under treatments with different opening sizes, levels of competition, knapweed seed burial and season of seeding. There was no effect of opening size where competing natives were alive, but knapweed emergence in 5- and 15-cm openings was higher than 0-cm openings where natives were killed. Reducing competition reduced fall diffuse knapweed emergence, but did not affect spring emergence. Seed burial increased knapweed emergence, but the effect varied by season. Although diffuse knapweed emergence reached 35%, only four plants survived from 3,600 seeds. This native grassland did not prevent knapweed emergence or establishment, but both were so low that rapid knapweed invasion is unlikely.     

松嫩平原碱化草甸恢复演替系列的群落种子流分析

对松嫩平原碱化草甸恢复演替系列4个群落优势种种子源库、土壤种子库及幼苗库进行了比较,建立了种子流模型.结果表明:演替初期虎尾草群落优势种种子源库密度最大,为(446182±180455)粒.m-2,分别是演替中期星星草群落的7.2倍、演替后期星星草 羊草群落的11.4倍、演替顶极羊草群落的164.8倍.土壤种子库和幼苗库密度均以虎尾草群落最大,分别为(63650±14541)粒.m-2和(39160±15192)株.m-2,羊草群落最小,分别为(14310±7686)粒.m-2和(790±745)株.m-2,大体呈随着恢复演替进程而递减的趋势.全体演替系列群落输出的实生苗均以虎尾草为主,占79.8%～100%.在种子流中,优势种的种子源库向土壤种子库输入率在10%～35%之间,输出率差异则较大,虎尾草群落高达62.3%,但星星草群落和羊草群落均没有优势种的幼苗输出.